Thermal management for implantable wireless power transfer systems

ABSTRACT

Thermal management solutions for wireless power transfer systems are provided, which may include any number of features. In one embodiment, an implantable wireless power receiver includes at least one thermal layer disposed on an interior surface of the receiver configured to conduct heat from a central portion of the receiver towards edges of the receiver. The thermal layer can comprise, for example, a copper layer or a ceramic layer embedded in an acrylic polymer matrix. In some embodiments, a plurality of thermal channels can be formed within the receiver to transport heat from central regions of the receiver towards edges of the receiver via free convection. In yet another embodiment, a fluid pipe can be connected to the receiver and be configured to carry heat from the receiver to a location remote from the receiver. Methods of use are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/953,547, filed Jul. 29, 2013, titled “Thermal Management forImplantable Wireless Power Transfer Systems”, which claims the benefitof U.S. Provisional Patent Application No. 61/676,626, filed Jul. 27,2012, titled “Thermal Management for Implantable Wireless Power TransferSystems”, and U.S. Provisional Patent Application No. 61/790,556, filedMar. 15, 2013, titled “Thermal Management for Implantable Wireless PowerTransfer Systems”.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

FIELD

The field relates generally to resonant wireless power transfer systems,and more specifically to implantable resonant wireless power transfersystems.

BACKGROUND

Transferring power from an external device to an implanted devicewirelessly with a Transcutaneous Energy Transfer (TET) system via, e.g.,an oscillating magnetic field, generates heat, which can cause atemperature rise in the device itself and/or in any surroundingmaterial.

Furthermore, an implanted device may require local electrical energystorage and processing when external power is interrupted or notavailable. This electrical energy processing and storage, typicallysolid-state electronics and a battery, generate heat while operating theimplant, and especially when the battery is charging or discharging. Forimplantable medical devices, the associated temperature rise of theimplant should be kept less than 2° C. to avoid tissue damage.

Many implantable medical devices, such as implanted sensors, requirevery little power to operate. With such low power levels (on the orderof milliwatts), temperature rise within the implant is not a concern asthe temperature rise easily remains below the 2° C. threshold. Withhigher power devices (e.g., on the order of watts and up to 15 W ormore), temperature rise is a larger concern.

Electrical devices that are powered wirelessly and require more powerare typically not implanted in human patients. For example, cell phonesand laptops can heat up much more than 2° C. when being charged, butthese temperatures are considered acceptable since they are notimplantable devices.

SUMMARY OF THE DISCLOSURE

A power receiver of a wireless power transfer system is provided,comprising an implantable housing, a receiver coil supported by theimplantable housing, electronics disposed within the implantable housingand coupled to the receiver coil, the electronics configured to controlwireless power transfer from an external power transmitter to thereceiver coil, and at least one thermal layer disposed on an interiorsurface of the implantable housing, the at least one thermal layerconfigured to conduct heat from a central portion of the implantablehousing towards edges of the implantable housing.

In some embodiments, the at least one thermal layer comprises at leastone copper layer. In another embodiment, the at least one thermal layercomprises a pair of thermal layers disposed on opposing internal sidesof the implantable housing.

In one embodiment, the at least one thermal layer comprises a thicknessof less than 0.5 mm copper. In another embodiment, the at least onethermal layer comprises a thickness of less than 1 mm aluminum.

In some embodiments, a central portion of the at least one thermal layercomprises a silicon rubber material. In another embodiment, the at leastone thermal layer comprises a copper layer having a central portioncomprising a silicon rubber material.

In some embodiments, the at least one thermal layer comprises a ceramiclayer embedded in an acrylic polymer matrix.

In another embodiment, the at least one thermal layer has a thickness ofless than 0.5 mm.

A power receiver of a wireless power transfer system is also provided,comprising an implantable housing, a receiver coil supported by theimplantable housing, at least one power converter disposed within theimplantable housing and coupled to the receiver coil, a plurality ofelectronics disposed within the implantable housing and coupled to thereceiver coil and the power converter, the electronics configured tocontrol wireless power transfer from an external power transmitter tothe at least one power converter, and a plurality of thermal channelsformed by an arrangement of the at least one power converter and theplurality of electronics in the implantable housing, the plurality ofthermal channels configured to allow a thermal medium to transport heatfrom central regions of the implantable housing towards edges of theimplantable housing via free convection.

In some embodiments, the thermal channels are arranged vertically withrespect to gravity within the implantable housing.

In another embodiment, the power receiver comprises at least one controlsurface disposed in the implantable housing to further define theplurality of thermal channels.

In some embodiments, the thermal medium comprises a fluid. In otherembodiments, the thermal medium comprises a gas.

A power receiver of a wireless power transfer system is provided,comprising a housing, a receiver coil supported by the housing, aplurality of electronics disposed within the housing and coupled to thereceiver coil, the electronics configured to control wireless powertransfer from an external power transmitter to the receiver coil, athermal medium disposed in the housing, the thermal medium having aboiling point at a temperature greater than 38° C. but less than 45° C.,and a fluid pipe coupled to an interior of the housing, the fluid pipeconfigured to carry a vapor generated by the thermal medium to alocation remote from the housing when the thermal medium surpasses itsboiling point in response to heat generated in the housing.

In some embodiments, the location remote from the housing is animplantable medical device coupled to the fluid pipe.

In one embodiment, the medical device comprises a blood pump.

In some embodiments, heat from the vapor is rejected into a blood streamof a patient.

In one embodiment, the vapor is allowed to condense at the locationremote from the housing to form droplets which can be pulled backtowards the power receiver to absorb heat.

In another embodiment, the fluid pipe comprises a flexible tube.

A method of reducing heat transferred to tissue by an implanted wirelesspower receiver is provided, comprising the steps of implanting awireless power receiver in a patient, receiving wireless power with thewireless power receiver, and conducting heat from a central portion ofthe wireless power receiver towards edges of the wireless power receiverwith at least one thermal layer disposed on an interior surface of thewireless power receiver.

In some embodiments, the at least one thermal layer comprises at leastone copper layer.

In another embodiment, the at least one thermal layer comprises a pairof thermal layers disposed on opposing internal sides of the implantablehousing.

In some embodiments, the at least one thermal layer comprises athickness of less than 0.5 mm copper. In one embodiment, the at leastone thermal layer comprises a thickness of less than 1 mm aluminum. Inanother embodiment, a central portion of the at least one thermal layercomprises a silicon rubber material. In another embodiment, the at leastone thermal layer comprises a copper layer having a central portioncomprising a silicon rubber material.

A method of reducing heat transferred to tissue by an implanted wirelesspower receiver is provided, comprising the steps of implanting awireless power receiver in a patient, receiving wireless power with thewireless power receiver, and conducting heat from a central portion ofthe wireless power receiver towards edges of the wireless power receiverwith a flow of a thermal medium through a plurality of thermal channelsformed by an arrangement of electronics and/or batteries in the wirelesspower receiver.

In one embodiment, the thermal medium transports heat through thethermal channels via free convection. In another embodiment, the thermalmedium comprises a fluid or a gas.

A method of reducing heat transferred to tissue by an implanted wirelesspower receiver is provided, comprising the steps of implanting awireless power receiver in a patient, receiving wireless power with thewireless power receiver; heating a thermal medium disposed in thewireless power receiver in response to the receiving wireless powerstep, and carrying vapor generated by heating the thermal medium to alocation remote from the wireless power receiver with a fluid pipecoupled to the wireless power receiver.

In some embodiments, the location remote from the wireless powerreceiver comprises an implantable medical device coupled to the fluidpipe.

In another embodiment, the medical device comprises a blood pump.

In one embodiment, heat from the vapor is rejected into a blood streamof a patient.

In another embodiment, the vapor is allowed to condense at the locationremote from the wireless power receiver to form droplets which can bepulled back towards the wireless power receiver to absorb heat.

In some embodiments, the fluid pipe comprises a flexible tube that isrouted through the patient to the location remote from the wirelesspower receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 illustrates a basic wireless power transfer system.

FIG. 2 illustrates the flux generated by a pair of coils.

FIGS. 3A-3B illustrate the effect of coil alignment on the couplingcoefficient.

FIGS. 4A-4C illustrate various embodiments of a TETS implant having athermal management system.

FIG. 5 illustrates another embodiment of a TETS implant having a thermalmanagement system.

FIG. 6 illustrates yet another embodiment of a TETS implant having athermal management system.

DETAILED DESCRIPTION

In the description that follows, like components have been given thesame reference numerals, regardless of whether they are shown indifferent embodiments. To illustrate an embodiment(s) of the presentdisclosure in a clear and concise manner, the drawings may notnecessarily be to scale and certain features may be shown in somewhatschematic form. Features that are described and/or illustrated withrespect to one embodiment may be used in the same way or in a similarway in one or more other embodiments and/or in combination with orinstead of the features of the other embodiments.

Various aspects of the invention are similar to those described inInternational Patent Pub. No. WO2012045050; U.S. Pat. Nos. 8,140,168;7,865,245; 7,774,069; 7,711,433; 7,650,187; 7,571,007; 7,741,734;7,825,543; 6,591,139; 6,553,263; and 5,350,413; and U.S. Pub. Nos.2010/0308939; 2008/027293; and 2010/0102639, the entire contents ofwhich patents and applications are incorporated herein for all purposes.

Wireless Power Transmission System

Power may be transmitted wirelessly by magnetic induction. In variousembodiments, the transmitter and receiver are closely coupled.

In some cases “closely coupled” or “close coupling” refers to a systemthat requires the coils to be very near each other in order to operate.In some cases “loosely coupled” or “loose coupling” refers to a systemconfigured to operate when the coils have a significant spatial and/oraxial separation, and in some cases up to distance equal to or less thanthe diameter of the larger of the coils. In some cases, “looselycoupled” or “loose coupling” refers a system that is relativelyinsensitive to changes in physical separation and/or orientation of thereceiver and transmitter.

In various embodiments, the transmitter and receiver are non-resonantcoils. For example, a change in current in one coil induces a changingmagnetic field. The second coil within the magnetic field picks up themagnetic flux, which in turn induces a current in the second coil. Anexample of a closely coupled system with non-resonant coils is describedin International Pub. No. WO2000/074747, incorporated herein for allpurposes by reference. A conventional transformer is another example ofa closely coupled, non-resonant system. In various embodiments, thetransmitter and receiver are resonant coils. For example, one or both ofthe coils is connected to a tuning capacitor or other means forcontrolling the frequency in the respective coil. An example of closelycoupled system with resonant coils is described in International Pub.Nos. WO2001/037926; WO2012/087807; WO2012/087811; WO2012/087816;WO2012/087819; WO2010/030378; and WO2012/056365, and U.S. Pub. No.2003/0171792, incorporated herein for all purposes by reference.

In various embodiments, the transmitter and receiver are looselycoupled. For example, the transmitter can resonate to propagate magneticflux that is picked up by the receiver at relatively great distances. Insome cases energy can be transmitted over several meters. In a looselycoupled system power transfer may not necessarily depend on a criticaldistance. Rather, the system may be able to accommodate changes to thecoupling coefficient between the transmitter and receiver. An example ofa loosely coupled system is described in International Pub. No.WO2012/045050, incorporated herein for all purposes by reference.

Power may be transmitted wirelessly by radiating energy. In variousembodiments, the system comprises antennas. The antennas may be resonantor non-resonant. For example, non-resonant antennas may radiateelectromagnetic waves to create a field. The field can be near field orfar field. The field can be directional. Generally far field has greaterrange but a lower power transfer rate. An example of such a system forradiating energy with resonators is described in International Pub. No.WO2010/089354, incorporated herein for all purposes by reference. Anexample of such a non-resonant system is described in International Pub.No. WO2009/018271, incorporated herein for all purposes by reference.Instead of antenna, the system may comprise a high energy light sourcesuch as a laser. The system can be configured so photons carryelectromagnetic energy in a spatially restricted, direct, coherent pathfrom a transmission point to a receiving point. An example of such asystem is described in International Pub. No. WO2010/089354,incorporated herein for all purposes by reference.

Power may also be transmitted by taking advantage of the material ormedium through which the energy passes. For example, volume conductioninvolves transmitting electrical energy through tissue between atransmitting point and a receiving point. An example of such a system isdescribed in International Pub. No. WO2008/066941, incorporated hereinfor all purposes by reference.

Power may also be transferred using a capacitor charging technique. Thesystem can be resonant or non-resonant. Exemplars of capacitor chargingfor wireless energy transfer are described in International Pub. No.WO2012/056365, incorporated herein for all purposes by reference.

The system in accordance with various aspects of the invention will nowbe described in connection with a system for wireless energy transfer bymagnetic induction. The exemplary system utilizes resonant powertransfer. The system works by transmitting power between the twoinductively coupled coils. In contrast to a transformer, however, theexemplary coils are not coupled together closely. A transformergenerally requires the coils to be aligned and positioned directlyadjacent each other. The exemplary system accommodates looser couplingof the coils.

While described in terms of one receiver coil and one transmitter coil,one will appreciate from the description herein that the system may usetwo or more receiver coils and two or more transmitter coils. Forexample, the transmitter may be configured with two coils—a first coilto resonate flux and a second coil to excite the first coil. One willfurther appreciate from the description herein that usage of “resonator”and “coil” may be used somewhat interchangeably. In various respects,“resonator” refers to a coil and a capacitor connected together.

In accordance with various embodiments of this disclosure, the systemcomprises one or more transmitters configured to transmit powerwirelessly to one or more receivers. In various embodiments, the systemincludes a transmitter and more than one receiver in a multiplexedarrangement. A frequency generator may be electrically coupled to thetransmitter to drive the transmitter to transmit power at a particularfrequency or range of frequencies. The frequency generator can include avoltage controlled oscillator and one or more switchable arrays ofcapacitors, a voltage controlled oscillator and one or more varactors, aphase-locked-loop, a direct digital synthesizer, or combinationsthereof. The transmitter can be configured to transmit power at multiplefrequencies simultaneously. The frequency generator can include two ormore phase-locked-loops electrically coupled to a common referenceoscillator, two or more independent voltage controlled oscillators, orcombinations thereof. The transmitter can be arranged to simultaneouslydelivery power to multiple receivers at a common frequency.

In various embodiments, the transmitter is configured to transmit a lowpower signal at a particular frequency. The transmitter may transmit thelow power signal for a particular time and/or interval. In variousembodiments, the transmitter is configured to transmit a high powersignal wirelessly at a particular frequency. The transmitter maytransmit the high power signal for a particular time and/or interval.

In various embodiments, the receiver includes a frequency selectionmechanism electrically coupled to the receiver coil and arranged toallow the resonator to change a frequency or a range of frequencies thatthe receiver can receive. The frequency selection mechanism can includea switchable array of discrete capacitors, a variable capacitance, oneor more inductors electrically coupled to the receiving antenna,additional turns of a coil of the receiving antenna, or combinationsthereof.

In general, most of the flux from the transmitter coil does not reachthe receiver coil. The amount of flux generated by the transmitter coilthat reaches the receiver coil is described by “k” and referred to asthe “coupling coefficient.”

In various embodiments, the system is configured to maintain a value ofk in the range of between about 0.2 to about 0.01. In variousembodiments, the system is configured to maintain a value of k of atleast 0.01, at least 0.02, at least 0.03, at least 0.04, at least 0.05,at least 0.1, or at least 0.15.

In various embodiments, the coils are physically separated. In variousembodiments, the separation is greater than a thickness of the receivercoil. In various embodiments, the separation distance is equal to orless than the diameter of the larger of the receiver and transmittercoil.

Because most of the flux does not reach the receiver, the transmittercoil must generate a much larger field than what is coupled to thereceiver. In various embodiments, this is accomplished by configuringthe transmitter with a large number of amp-turns in the coil.

Since only the flux coupled to the receiver gets coupled to a real load,most of the energy in the field is reactive. The current in the coil canbe sustained with a capacitor connected to the coil to create aresonator. The power source thus only needs to supply the energyabsorbed by the receiver. The resonant capacitor maintains the excessflux that is not coupled to the receiver.

In various embodiments, the impedance of the receiver is matched to thetransmitter. This allows efficient transfer of energy out of thereceiver. In this case the receiver coil may not need to have a resonantcapacitor.

Turning now to FIG. 1, a simplified circuit for wireless energytransmission is shown. The exemplary system shows a series connection,but the system can be connected as either series or parallel on eitherthe transmitter or receiver side.

The exemplary transmitter includes a coil Lx connected to a power sourceVs by a capacitor Cx. The exemplary receiver includes a coil Lyconnected to a load by a capacitor Cy. Capacitor Cx may be configured tomake Lx resonate at a desired frequency. Capacitance Cx of thetransmitter coil may be defined by its geometry. Inductors Lx and Ly areconnected by coupling coefficient k. Mxy is the mutual inductancebetween the two coils. The mutual inductance, Mxy, is related tocoupling coefficient, k.Mxy=k√{square root over (Lx·Ly)}

In the exemplary system the power source Vs is in series with thetransmitter coil Lx so it may have to carry all the reactive current.This puts a larger burden on the current rating of the power source andany resistance in the source will add to losses.

The exemplary system includes a receiver configured to receive energywirelessly transmitted by the transmitter. The exemplary receiver isconnected to a load. The receiver and load may be connected electricallywith a controllable switch.

In various embodiments, the receiver includes a circuit elementconfigured to be connected or disconnected from the receiver coil by anelectronically controllable switch. The electrical coupling can includeboth a serial and parallel arrangement. The circuit element can includea resistor, capacitor, inductor, lengths of an antenna structure, orcombinations thereof. The system can be configured such that power istransmitted by the transmitter and can be received by the receiver inpredetermined time increments.

In various embodiments, the transmitter coil and/or the receiver coil isa substantially two-dimensional structure. In various embodiments, thetransmitter coil may be coupled to a transmitter impedance-matchingstructure. Similarly, the receiver coil may be coupled to a receiverimpedance-matching structure. Examples of suitable impedance-matchingstructures include, but are not limited to, a coil, a loop, atransformer, and/or any impedance-matching network. Animpedance-matching network may include inductors or capacitorsconfigured to connect a signal source to the resonator structure.

In various embodiments, the transmitter is controlled by a controller(not shown) and driving circuit. The controller and/or driving circuitmay include a directional coupler, a signal generator, and/or anamplifier. The controller may be configured to adjust the transmitterfrequency or amplifier gain to compensate for changes to the couplingbetween the receiver and transmitter.

In various embodiments, the transmitter coil is connected to animpedance-matched coil loop. The loop is connected to a power source andis configured to excite the transmitter coil. The first coil loop mayhave finite output impedance. A signal generator output may be amplifiedand fed to the transmitter coil. In use power is transferredmagnetically between the first coil loop and the main transmitter coil,which in turns transmits flux to the receiver. Energy received by thereceiver coil is delivered by Ohmic connection to the load.

One of the challenges to a practical circuit is how to get energy in andout of the resonators. Simply putting the power source and load inseries or parallel with the resonators is difficult because of thevoltage and current required. In various embodiments, the system isconfigured to achieve an approximate energy balance by analyzing thesystem characteristics, estimating voltages and currents involved, andcontrolling circuit elements to deliver the power needed by thereceiver.

In an exemplary embodiment, the system load power, P_(L), is assumed tobe 15 Watts and the operating frequency, f, is 250 kHz. Then, for eachcycle the load removes a certain amount of energy from the resonance:

${e_{L} = {\frac{P_{L}}{f} = {60\mspace{14mu}\mu\; J}}}{Energy}\mspace{14mu}{the}\mspace{14mu}{load}\mspace{14mu}{removes}\mspace{14mu}{in}\mspace{14mu}{one}\mspace{14mu}{cycle}$

It has been found that the energy in the receiver resonance is typicallyseveral times larger than the energy removed by the load for operative,implantable medical devices. In various embodiments, the system assumesa ratio 7:1 for energy at the receiver versus the load removed. Underthis assumption, the instantaneous energy in the exemplary receiverresonance is 420 μJ.

The exemplary circuit was analyzed and the self inductance of thereceiver coil was found to be 60 uH. From the energy and the inductance,the voltage and current in the resonator could be calculated.

$e_{y} = {\frac{1}{2}{Li}^{2}}$$i_{y} = {\sqrt{\frac{2e_{y}}{L}} = {3.74\mspace{14mu} A\mspace{14mu}{peak}}}$v_(y) = ω L_(y)i_(y) = 352  V  peak

The voltage and current can be traded off against each other. Theinductor may couple the same amount of flux regardless of the number ofturns. The Amp-turns of the coil needs to stay the same in this example,so more turns means the current is reduced. The coil voltage, however,will need to increase. Likewise, the voltage can be reduced at theexpense of a higher current. The transmitter coil needs to have muchmore flux. The transmitter flux is related to the receiver flux by thecoupling coefficient. Accordingly, the energy in the field from thetransmitter coil is scaled by k.

$e_{x} = \frac{e_{y}}{k}$

Given that k is 0.05:

$e_{x} = {\frac{420\mspace{14mu}\mu\; J}{0.05} = {8.4\mspace{14mu}{mJ}}}$

For the same circuit the self inductance of the transmitter coil was 146uH as mentioned above. This results in:

$i_{x} = {\sqrt{\frac{2e_{x}}{L}} = {10.7\mspace{14mu} A\mspace{14mu}{peak}}}$v_(x) = ω L_(x)i_(x) = 2460  V  peak

One can appreciate from this example, the competing factors and how tobalance voltage, current, and inductance to suit the circumstance andachieve the desired outcome. Like the receiver, the voltage and currentcan be traded off against each other. In this example, the voltages andcurrents in the system are relatively high. One can adjust the tuning tolower the voltage and/or current at the receiver if the load is lower.

Estimation of Coupling Coefficient and Mutual Inductance

As explained above, the coupling coefficient, k, may be useful for anumber of reasons. In one example, the coupling coefficient can be usedto understand the arrangement of the coils relative to each other sotuning adjustments can be made to ensure adequate performance. If thereceiver coil moves away from the transmitter coil, the mutualinductance will decrease, and ceteris paribus, less power will betransferred. In various embodiments, the system is configured to maketuning adjustments to compensate for the drop in coupling efficiency.

The exemplary system described above often has imperfect information.For various reasons as would be understood by one of skill in the art,the system does not collect data for all parameters. Moreover, becauseof the physical gap between coils and without an external means ofcommunications between the two resonators, the transmitter may haveinformation that the receiver does not have and vice versa. Theselimitations make it difficult to directly measure and derive thecoupling coefficient, k, in real time.

Described below are several principles for estimating the couplingcoefficient, k, for two coils of a given geometry. The approaches maymake use of techniques such as Biot-Savart calculations or finiteelement methods. Certain assumptions and generalizations, based on howthe coils interact in specific orientations, are made for the sake ofsimplicity of understanding. From an electric circuit point of view, allthe physical geometry permutations can generally lead to the couplingcoefficient.

If two coils are arranged so they are in the same plane, with one coilcircumscribing the other, then the coupling coefficient can be estimatedto be roughly proportional to the ratio of the area of the two coils.This assumes the flux generated by coil 1 is roughly uniform over thearea it encloses as shown in FIG. 2.

If the coils are out of alignment such that the coils are at a relativeangle, the coupling coefficient will decrease. The amount of thedecrease is estimated to be about equal to the cosine of the angle asshown in FIG. 3A. If the coils are orthogonal to each other such thattheta (θ) is 90 degrees, the flux will not be received by the receiverand the coupling coefficient will be zero.

If the coils are arraigned such that half the flux from one coil is inone direction and the other half is in the other direction, the fluxcancels out and the coupling coefficient is zero, as shown in FIG. 3B.

A final principle relies on symmetry of the coils. The couplingcoefficient and mutual inductance from one coil to the other is assumedto be the same regardless of which coil is being energized.M _(xy) =M _(yx)

Heat generated by implanted medical devices can cause a temperatureincrease in surrounding tissue. Table 1 below contains data derived fromtests conducted to determine effects on different tissue types due toheat generated by implanted devices. The devices were implanted invarious sites within the animal (as noted in column 2) in contact withdifferent tissue types, and the effective thermal conductivity wascalculated as shown in column 3 using an adjusted thermal conductivitythat enables the inclusion of convection (in addition to conduction)when using a diffusion equation. Column 4 shows the device power settingat each site. The devices were powered for 30-60 minutes. Column 5 showsthe calculated temperature rise at the hottest point at each site if theimplanted device had been left powered on for many hours rather than the30-60 minutes of each test. In some cases, the effective thermalconductivity was different on different sides of the implanted module,because the module was located between two different types of tissue.This is noted in the table where appropriate.

TABLE 1 k_(eff) P t→∞: max ΔT Location [W/m · ° C.] [W] [° C.] Box A1 Rchest between skin (1.5 cm) 0.66 −/+ 0.05 (skin) 5 8.0 +/− 0.4 andmuscles (20 cm) 0.47 −/+ 0.05 (muscle) Box B3 R chest between 2 −/+ 0.05(lung) 5 lung (20 cm)and ribs (>5 cm) Puck A2 R pectoral between skin0.25 −/+ 0.01 0.7 (coil) + 4.0 +/− 0.1 (D01) (1 cm) and muscles (20 cm)0.3 (caps) Puck B1 L posterior between muscles 0.53 −/+ 0.01 0.7(coil) + 2.9 +/− 0.1 (D03) (2.5 cm and >6 cm) 0.3 (caps) Puck B2 Labdominal wall under rectus 0.36 −/+ 0.01 0.7 (coil) + 3.0 +/− 0.1 (D02)muscle (1.5 cm and ~20 cm) 0.3 (caps)

Systems and methods are described herein for reducing and minimizingtemperature rise in an implantable medical device resulting fromwireless or Transcutaneous Energy Transfer System (TETS) power transfer.In some embodiments, thermal management within an implantable medicaldevice is accomplished by limiting the temperature rise on the surfaceof the enclosure that contacts human tissue. In other embodiments,thermal management within an implantable medical device is accomplishedby utilizing a thermal fluid or gas such as helium gas or transformeroil within the implant. In additional embodiments, thermal managementcan be accomplished using a thermal medium within the implantablemedical device and a fluid pipe configured to carry vapor generated bythe thermal medium away from the implant.

FIG. 4A is an external view of a TETS implant 100, comprising an outerhousing 102. The TETS implant 100 can comprise an implantable receiverconfigured to receive wireless energy from a transmitter external to thepatient. In some embodiments, the housing can be in the shape of a cube,with six equally shaped sides. In another embodiment, as shown in FIG.4A, the housing 102 can comprise a cuboid shape that includes two“large” sides 110 and four “small” sides 112. Also shown in FIG. 4A is areceive coil 105 which can be configured to wirelessly receive powertransmitted from an external source, such as from a transmit coil (notshown).

In some embodiments, the TETS implant can have exterior dimensions ofapproximately 12 cm by 10 cm by 3.5 cm, and a weight of approximately500 g. In another embodiment, the implant can be 20 cm by 10 cm by 1.7cm and slightly curved around the 10 cm axis, or cylindricalapproximately 8 cm diameter and 1.5 cm thick. The TETS implant 100 canbe a power receiver configured to communicate with a power transmitterfor wireless power transmission.

FIG. 4B is a cross-sectional view of TETS implant 100, showing the outerhousing 102 and an optional inner housing 104. The inner housing 104 canbe sized and shaped to conform to the inner walls of outer housing 102.As described above, the TETS implant can be configured for implantationinto the human body and can include, for example, housings 102 and 104that contains and/or supports all components needed for wireless energytransfer, such as the such as the receive coil and electronics 106 andcircuitry configured to manage and control the transfer of power fromthe receive coil to a battery 107 in the implant. In one embodiment, theelectronics can include at least one power converter disposed within thehousing and coupled to the receiver coil. The electronics can beconfigured to control wireless power transfer from an external powertransmitter to the at least one power converter and/or electronics. TheTETS implant 100 can be optionally connected to other implanted medicaldevices within the body to provide power for those devices. In oneembodiment, outer housing 102 can comprise ferrimagnetic orferromagnetic material, such as MnZn ferrite, and inner housing 104 cancomprise titanium. Receive coil 105 of FIG. 4A is not shown in FIG. 4Bfor ease of illustration.

Heat transfer inside the human body is dominated by conduction andconvection. These processes act on temperature gradients in such a wayas to round out the thermal profile, i.e., any iso-surface of constanttemperature that has a high aspect ratio will, over time, become moreand more spherical. Mathematically, this is particularly obvious fromthe diffusion equation, which states that the time derivative of thetemperature is proportional to the second spatial derivative of thetemperature. The diffusion equation governs heat conduction, and nearthe surface of an implanted device, convection can also be modeled withthe diffusion equation through the use of an effective thermalconductivity tensor.

The TETS implant 100 of FIGS. 4A-4B can include a high aspect ratiobetween the sizes of the sides of the cuboid in order to maximize thearea of the receive coil that wraps around the housing. Referring toFIG. 4A, the implant can have a relatively large length “L” and width“W” with respect to the depth “D” of the implant. Therefore, thecross-section of implant 100 can include two “large” sides 110 (withdimensions L×W) and four “small” sides 112 (with dimensions D×L or D×W).In this type of implant, heat generated inside the housing will raisethe temperature the most at the center of the large sides 110.

Referring to FIG. 4B, to manage heat generated within implant 100,portions of the interior of the implant housing can be lined with atleast one thermal layer 108 to dissipate heat generated by electronics106 and battery 107. In the example shown in FIG. 4B, thermal layer 108is disposed on the inside of housing, along the large sides 110. In theembodiment of FIG. 4B, a small gap or space is illustrated between theelectronics 106 and battery 107 and the thermal layer 108. However, inother embodiments, the thermal layer 108 is positioned directly againstthe electronics 106 and/or battery 107.

In some embodiments, the thermal layers can comprise materials with goodheat conductivity, such as copper, ceramics, silicon, rubber, silver,aluminum, beryllium, other noble metals, and even exotic materials suchas diamond. However, copper is likely to be one of the mostcost-effective materials with good thermal conductivity. The layer canbe less than approximately 0.5 mm thick, and can comprise a flat sheet.A pair of 0.5 mm thick copper thermal layers disposed in the implant ofFIG. 4B can reduce the maximum tissue temperature by approximately 1°C., for example. In one specific embodiment, the thermal layer cancomprise a highly thermally conductive ceramic embedded in an acrylicpolymer matrix, having an overall thickness of less than or equal to0.25 mm and thermal conductivity of approximately 0.6 W/mK. Thisembodiment provides thermal conductivity, electrical isolation, and anadhesive layer for bonding internal components together

By placing the thermal layers along the entire inner walls of sides 110of the TETS implant, heat is lead from the center of the housing towardsthe edges of the housing. Heat release at or near the edges or cornersof the housing is free to expand into a three-dimensional space, causinga lower temperature rise in the tissue of the patient than heat releasednear the center of the housing, where heat essentially expandsone-dimensionally. It is important to note that only lining the centerportion of sides 110 would not be as effective as lining the entireinner sides of the housing, since heat expansion would remainessentially one-dimensional.

In some embodiments, the electrical components within the housing can bepurposefully placed away from the center of sides 110; however this maynot be practical given the space constraints of the implant.

In another embodiment, shown in FIG. 4C, the thermal layer 108 caninclude a central portion 109 that can comprise, for example a thermalinsulator such as a silicone rubber material. Utilizing a centralportion 109 having a different material than thermal layer 108 candirect heat away from the center of sides 110 and towards the edges ofthe TETS implant. In one embodiment, central portion 109 can have alower thermal conductivity than thermal layer 108 if it is a homogeneousmaterial. In another embodiment, the central portion 109 can have ahigher thermal conductivity in an axis connecting central portion 109 tothermal layer 108 than in an axis connecting central portion 109 toinner housing 104, which would promote heat spreading towards the edgesof the housing.

FIG. 5 illustrates another TETS implant 200 that utilizes anotherstructure and technique for thermal management. Implant 200 can includehousings 202 and 204, similar to housings 102 and 104 described above inFIGS. 4A-4B. Electronics 206 including battery 207 can be disposedwithin the housings of implant 200. Electronics 206 can be similar tothe electronics described above and can include at least one powerconverter. In the embodiment shown in FIG. 5, electronics 206 andbattery 207 can be arranged within the housing as shown to form aplurality of thermal channels 216 between the individual battery andelectronics components. In FIG. 5, the channels 216 are formed by twoseparate electronics 206 and batteries 207, however it should beunderstood that not all implants will have this number ofbatteries/electronics. In some embodiments, only the electronics andbatteries are used to form the thermal channels. However, in otherembodiments, control surfaces (not shown) can be utilized to form thechannels, wherein the control surfaces only purpose within the implantis to form the channels.

In the embodiment of FIG. 5, to minimize or reduce temperature rise onthe surface of the implant due to wireless power transfer, implant 200can be filled with a thermal medium 214, and the thermal medium can beallowed to flow freely along the thermal channels 216 formed by thespacing of the electronics and batteries or control surfaces. In someembodiments, the thermal medium 214 can be either a fluid or a gas witha significant heat capacity and thermal expansion coefficient that doesnot change sign in a temperature interval of interest. In someembodiments, the temperature interval of interest ranges from about 37°C. to 50° C. The thermal medium can comprise a low viscosity and a lowdensity for reduced weight. Additionally, a low surface adhesion isdesirable so that it does not wick to surfaces, and a high electricalresistivity is desired so as to be electrically inert. One example of asuitable thermal medium would be Fluorinert™ from 3M.

The thermal channels can be oriented to allow the thermal medium totransport heat from the central regions of the implant towards the edgesof the housing, due to thermal expansion of the thermal medium and freeconvection. In some embodiments, the flow paths of the thermal mediumare shown by the arrows within the fluid channels of FIG. 5. In theembodiment of FIG. 5, the fluid channels are shown as being arrangedhorizontally and being substantially parallel, however, when implantedin a patient, the implant would typically be arranged so that the fluidchannels are arranged vertically to facilitate convective flow. Whenarranged vertically, thermal fluid in the central channels would expandand rise up when heated, then cool and flow back down along the sidechannels. In other embodiments the fluid channels can comprise zigzag,curved, or other unpredictable patterns to extend the length the mediumtraverses when near the internal walls of the implant. In someembodiments, the fluid flow does not rely on gravity for freeconvection, but instead can be driven within the implant by a pumpingsystem disposed in the implant.

FIG. 6 illustrates yet another embodiment of a TETS implant 300 having athermal management system. In FIG. 6, the implant 300 can includehousings 302 and 304, electronics 306, and battery 307, similar tohousings 102 and 104, electronics 106, and battery 107 described above.The implant can also include a receive coil similar to the coil of FIG.4A configured for receiving wireless power from an external transmitcoil. The interior of implant 300 can be filled with a thermal medium314. In some embodiments, the thermal medium 314 is chosen to have aboiling point at a temperature greater than 38° C. The thermal mediumcan comprise a fluid having a low viscosity and a low density forreduced weight. Additionally, a low surface adhesion is desirable sothat it does not wick to surfaces, and a high electrical resistivity isdesired so as to be electrically inert. Additionally, fluid pipe 318 canbe coupled on one end to the interior of the implant and on another endto an implanted medical device 320, such as a blood pump. Fluid pipe 318can be a flexible tube configured to conform to the body's internalorgans during bodily movement. In some embodiments, the fluid pipe canbe constructed of an open-cell foam structure.

When the TETS implant 300 generates heat, such as during operation orduring wireless power transfer or battery charging, the thermal medium314 will also rise in temperature. As the thermal medium approaches andsurpasses its boiling point, the thermal medium will naturally boil andcreate vapor. The vapor can be carried vertically by fluid pipe 318 todevice 320. When the implant is implanted within a patient, the fluidpipe can be oriented towards the topmost portion of the implant to placeit at the furthest point away from gravity. Because of the high energyabsorbed during phase change without a rise in temperature, all internalcomponents of the TETS implant and the housings themselves can be keptat or below the boiling temperature of the thermal medium. As the vaporis delivered to the implanted medical device 320, the vapors cancondense to form droplets, which can then be pulled by gravity or thefluid pump back towards the TETS implant to absorb more heat.

Instead of dissipating heat to tissue surrounding the TETS implant 300,the heat can be rejected via the fluid pipe towards the other implantedmedical device. In the instance where the medical device comprises ablood pump, the heat can be rejected into the blood itself with a knownflow rate, set by the pump speed and power, and known thermalproperties. This approach allows for higher power electronics enclosedwithin a smaller volume of TETS implant 300. It also allows theelectronics to overcome transient thermal events, such as a momentaryhigh current battery charging operation, without exceeding an enclosuresurface temperature specification. Without a thermal medium, eitherthese electronics components would overheat, or the heat generationwould cause some portions of the enclosure to rise by more than 2° C.,risking damage to local tissue. Additionally, the thermal managementsystem described with respect to FIG. 6 does not require additionalthermal management features, such as external fins or internal heatdiffusers, allowing the TETS implant to be smaller. This design canresult in almost no heat being rejected to the tissue surrounding theTETS implant.

It is important to note that in this embodiment, the implanted medicaldevice 320 must be positioned in the patient's body higher than theimplanted position of the TETS implant 300. If the patient orientateshimself in a way that puts the implanted medical device 320 below theTETS implant (e.g., the patient lies down with his head below his feet),the system described herein cannot rely on phase change cooling.Instead, the thermal medium must completely fill the fluid pipe betweenthe TETS implant and the implanted medical device. In this embodiment,the TETS implant can be fitted with a pump to actively move the thermalfluid through the fluid pipe.

In yet another embodiment, a thermal sheet could be placed around thebox to act as a “heat tube.” The lungs of a patient are a large heatconductor, so in some embodiments the thermal sheet could be bent by asurgeon to conform to a lung at the end of an exhale cycle, and theimplant could dissipate heat through the sheet into the lungs of thepatient.

As for additional details pertinent to the present invention, materialsand manufacturing techniques may be employed as within the level ofthose with skill in the relevant art. The same may hold true withrespect to method-based aspects of the invention in terms of additionalacts commonly or logically employed. Also, it is contemplated that anyoptional feature of the inventive variations described may be set forthand claimed independently, or in combination with any one or more of thefeatures described herein. Likewise, reference to a singular item,includes the possibility that there are plural of the same itemspresent. More specifically, as used herein and in the appended claims,the singular forms “a,” “and,” “said,” and “the” include pluralreferents unless the context clearly dictates otherwise. It is furthernoted that the claims may be drafted to exclude any optional element. Assuch, this statement is intended to serve as antecedent basis for use ofsuch exclusive terminology as “solely,” “only” and the like inconnection with the recitation of claim elements, or use of a “negative”limitation. Unless defined otherwise herein, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs. The breadth of the present invention is not to be limited bythe subject specification, but rather only by the plain meaning of theclaim terms employed.

What is claimed is:
 1. A power receiver of a wireless power transfersystem, comprising: an outer ferrite housing; an inner titanium housingdisposed within the outer ferrite housing; a receiver coil supported bythe outer ferrite housing; a plurality of electronics disposed withinthe inner titanium housing and coupled to the receiver coil, theelectronics configured to control wireless power transfer from anexternal power transmitter to the receiver coil; a thermal mediumdisposed in the inner titanium housing, the thermal medium having aboiling point at a temperature greater than 38° C. but less than 45° C.;and a pipe having a first end and a second end, the first end in flowcommunication with the thermal medium, the second end positioned at alocation exterior to the outer ferrite housing, the pipe extending fromthe first end to the second end through the inner titanium housing andthe outer ferrite housing, the pipe configured to carry a fluid or vaporgenerated by the thermal medium to the location exterior to the outerferrite housing when the thermal medium surpasses its boiling point inresponse to heat generated in the outer ferrite housing.
 2. The powerreceiver of claim 1 wherein the second end of the pipe is coupled to animplantable medical device.
 3. The power receiver of claim 2 wherein themedical device comprises a blood pump.
 4. The power receiver of claim 1wherein heat from the fluid or vapor is rejected into a blood stream ofa patient.
 5. The power receiver of claim 1 wherein the vapor is allowedto condense at the location exterior to the outer ferrite housing toform droplets which can be pulled back towards the power receiver toabsorb heat.
 6. The power receiver of claim 1 wherein the pipe comprisesa flexible tube.